The present invention relates to an optical scanner for use with a laser printer, a laser display and others.
Optical scanners may generally be classified into two different types, i.e., a pre-objective type and a post-objective type. Generally, in a pre-objective type scanner a light beam issuing from a light source is deflected at a constant angular velocity by an optical deflector which is implemented with a polygon mirror, the deflected beam scanning a photoconductive element or like object through an f.theta. lens at a constant scanning speed. In a post-objective type scanner, on the other hand, a plurality of clocks are seletected one after another to stepwisely change the frequency of a picture scanning clock, thereby eliminating the need for an f.theta. lens. An example of such post-objective type scanners is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 58-87965. In this instance, a light source is modulated by a data signal which is delivered responsive to the image scanning clock.
The problem with the ordinary pre-objective type scanner is that it is very expensive due to the use of an f.theta. lens. The post-objective type scanner which is constructed to compensate for scanning distortion by stepwisely varying the image scanning clock is disadvantageous in that the frequency variation of the clock is distcontinuous and, therefore, the scanning accuracy is limited. Assuming that the specific scanning distortion is 0.5 percent, for example, the accuracy at those points where the image scanning clock frequency is discontinuous cannot be controlled to the specific value without relying on a source clock which is 1/0.005=200 times higher than the image scanning clock frequency; where the image scanning clock frequency is 2 MHz, for example, the source clock frequency has to be 400 MHz which is impractically high.